A pressurized-water reactor's steam generator's heat-exchanger tube bundle must operate under the stress of high-velocity flow of the reactor pressurized-water coolant which is passed through the tube bundle, and under the stress involved by the flow of steam-generating feed water upwardly through the tube bundle outside of its tubes.
An example of a prior art steam generator, such as is currently used in the main coolant loop of a pressurized-water reactor, is provided by the Green U.S. Pat. No. 3,483,848, dated Dec. 16, 1969. This reference to this patent hereby incorporates its disclosure into the present specification.
As shown by the above patent, such a steam generator has a heat-exchanger formed by an inverted U-tube bundle formed by a large number of individual U-shaped tubes, the tube bundle being vertical with its tube ends mounted in a horizontal tube sheet through which the reactor coolant is passed via appropriate intake and outlet manifolds connected to the main coolant loop of the reactor. Flow velocities, temperatures and fluid pressures are all of a high order. The individual tubes of the tube bundle, particularly throughout their vertical leg portions, must be kept separated from each other by uniformly distributed interspacing.
To keep the individual tubes of such a heat-exchanger properly interspaced, the vertical legs are held by the grid bars of a tube spacer grid, this being formed by an annular grid frame for attachment to the inside of the steam generator's casing, and mounting appropriately interspaced, criss-crossed grid bars, forming a latticework having appropriately spaced openings through which the individual heat-exchanger tubes are arranged.
Heretofore, the grid bars have been welded directly to the grid frame. Thermal expansion and contraction of the grid bars has caused them to warp out of shape when in service. The welding operation required for the connection of the grid bar ends to the grid frame has sometimes caused local deformations. The application of such prior art spacer grids to the tube bundle legs has been troublesome and time consuming.